The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast that includes a circuit for quickly detecting a lamp-out condition.
Electronic ballasts that include an inverter and a series resonant type output circuit generally require some form of protection circuitry in order to prevent excessive power dissipation and/or damage due to the high voltages and currents that tend to result when a lamp fails or is removed. It is especially important that the protection circuitry quickly detect lamp failure or removal so that appropriate control action may be taken (e.g., shutting down the inverter) before the voltages and currents in the inverter and resonant circuit reach undesirably high levels.
There are many types of protection circuits in the prior art. These protection circuits may be classified according to the signals that are monitored in order to detect a lamp fault condition. In one group are xe2x80x9csupply-sidexe2x80x9d approaches that are concerned with monitoring signals in the inverter portion of the ballast, such as the current through the inverter switches which is usually monitored via a current-sensing resistor placed in series with one of the inverter switches. Such circuits are most readily implemented in ballasts with driven, as opposed to self-oscillating, inverters. In another group are xe2x80x9cload sidexe2x80x9d approaches that focus on signals at the ballast output and the lamp(s), such as the current that flows through the lamp(s) or the voltage that appears across a direct current (DC) blocking capacitor in series with the lamp(s). The present invention is intended as an alternative to existing approaches within this latter lass of protection circuits.
One known xe2x80x9cload sidexe2x80x9d approach employs either a current transformer or current-sensing resistor that is placed in series with the lamp(s) in order to directly monitor the lamp current. However, both of these components have significant drawbacks. A current transformer is quite costly in terms of both material and ballast manufacturability. A current sensing resistor, while materially inexpensive, is significantly dissipative and thus undesirable from the standpoint of ballast energy efficiency.
Another known xe2x80x9cload sidexe2x80x9d approach monitors the voltage across a direct current (DC) blocking capacitor in series with the lamp load. As illustrated in FIG. 1, a typical realization of this approach utilizes a resistor voltage divider arrangement (R1, R2) connected in parallel with the DC blocking capacitor (CB). The operation and limitations of this approach are discussed with reference to FIGS. 1 and 2 as follows.
During normal operation, when the lamp load is conducting current in a normal manner, the voltage across CB has an average value of VDC/2 (e.g., 225 volts). VOUT is a highly scaled-down version of the voltage across CB, and is typically set to have an average value that is on the order of several volts (e.g., 5 volts) when the lamp load is operating normally. For the sake of later comparison, it is assumed that the inverter drive circuit is configured to turn the inverter off (or take some other type of protective action) when VOUT falls below a predetermined value (e.g., 2.5 volts).
If the lamp load is removed or fails to conduct current, CB is deprived of charging current and begins to discharge into R1 and R2. Correspondingly, the voltage across CB, and hence VOUT, decreases. Once VOUT falls below a predetermined level (e.g., 2.5 volts), the inverter drive circuit senses that there is a lamp fault and takes appropriate control action (e.g., shuts down the inverter) in order to limit power dissipation and prevent damage to the ballast.
FIG. 2 is an approximate plot of VOUT for when the circuit of FIG. 1 is realized with the following component and parameter values: VDC=450 volts, CB=0.1 microfarad, ILAMP=180 milliamperes (rms), R1=220 kilohms, R2=5.1 kilohms. During the period 0 less than t less than t1, the lamp load is operating normally and the voltage across CB is at its normal value of VDC/2=225 volts. Correspondingly, VOUT has an average (DC) value of approximately 5 volts; VOUT also includes a small amount of high frequency ripple. Upon occurrence of a lamp-out condition (i.e., removal of the lamp or failure of the lamp to conduct current) at time t1, the voltage across CB begins to decrease as a rate determined by the capacitance of CB and the sum of the resistances of R1 and R2. After about 16 milliseconds, at t=t2, VOUT reaches about half (i.e., 2.5 volts) of its normal operating value (i.e., 5 volts), at which point the inverter drive circuit shuts down the inverter or shifts the inverter operating frequency to a value that is far enough removed from the natural resonant frequency of LR and CR so as to limit power dissipation and prevent undesirably high voltages and currents in the ballast.
In a real ballast, the inverter is normally operated at a frequency that is at or near the natural resonant frequency of LR and CR; for a number of practical reasons, this frequency is preferably set to be greater than 20,000 hertz. With such a high operating frequency, it does not take very long for the voltages and currents in the inverter and resonant circuit to reach damaging levels after a lamp fault occurs. For example, with an operating (and resonant) frequency of 40,000 hertz, the voltages and currents in the ballast will have reached undesirably levels within as few as 4-5 cycles (e.g., 100-125 microseconds) or so after occurrence of a lamp fault. Because 125 microseconds is far less than the 16 milliseconds that it takes for VOUT to fall to a level that indicates a lamp-out condition, this approach is not nearly fast enough to serve as a reliable protection circuit.
In the prior art circuit of FIG. 1, the time that it takes for VOUT to decrease by a given amount following a lamp-out condition is governed by CB, R1, and R2. Although the time may be shortened by decreasing the capacitance of CB and/or the sum of the resistances of R1 and R2, there are other constraints that render this strategy impractical. First, because the minimum required capacitance of CB is dictated by the magnitude of ILAMP and other design considerations, a reduction in the capacitance of CB is generally not an option. Second, in order to prevent life-shortening migration effects in the lamp(s) due to the presence of a direct current (DC) component in ILAMP, the sum of the resistances of R1 and R2 must be large enough to limit the DC component of ILAMP to no more than one milliampere during normal operation of the lamp load. With R1+R2 set to 225.1 kilohms and with VDC set to 450 volts (as in the present example), the DC component of ILAMP is approximately one milliampere. Any further reduction in R1+R2 would cause the DC component to exceed one milliampere, which would be unacceptable. Thus, there is no apparent way in which to shorten the response time of the approach of FIG. 1 without violating other important design constraints.
What is needed, therefore, is a ballast with a compact and cost-effective arrangement for quickly detecting and responding to lamp removal or failure, but without introducing excessive DC current through the lamps. A ballast with these features would represent a significant advance over the prior art.